Continuous deposition process and apparatus for manufacturing cadmium telluride photovoltaic devices

Abstract

A continuous deposition process and apparatus for depositing semiconductor layers containing cadmium, tellurium or sulfur as a principal constituent on transparent substrates to form photovoltaic devices as the substrates are continuously conveyed through the deposition apparatus is described. The film deposition process for a photovoltaic device having an n-type window layer and three p-type absorber layers in contiguous contact is carried out by a modular continuous deposition apparatus which has a plurality of processing stations connected in series for depositing successive layers of semiconductor films onto continuously conveying substrates. The fabrication starts by providing an optically transparent substrate coated with a transparent conductive oxide layer, onto which an n-type window layer formed of CdS or CdZnS is sputter deposited. After the window layer is deposited, a first absorber layer is deposited thereon by sputter deposition. Thereafter, a second absorber layer formed of CdTe is deposited onto the first absorber layer by a novel vapor deposition process in which the CdTe film forming vapor is generated by sublimation of a CdTe source material. After the second absorber layer is deposited, a third absorber layer formed of CdHgTe is deposited thereon by sputter deposition. The substrates are continuously conveyed through the modular continuous deposition apparatus as successive layers of semiconductor films are deposited thereon.

1. A method for fabricating a heterojunction photovoltaic device including three p-type absorber layers in contiguous contact, each of the three absorber layers having a different composition and containing cadmium as a principal constituent, the method comprising the steps of:
 (a) providing an optically transparent substrate coated with a transparent conductive oxide layer thereon; 
 (b) depositing an n-type window layer onto said transparent conductive oxide layer by sputter deposition; 
 (c) depositing a first p-type absorber layer onto said window layer by sputter deposition; 
 (d) depositing a second p-type absorber layer onto said first absorber layer; 
 (e) depositing a third p-type absorber layer onto said second absorber layer by sputter deposition; 
 (f) depositing a CdCl 2  layer onto said third absorber layer; and 
 (g) annealing said CdCl 2  layer and said absorber layers at a temperature in the range of from about 380° C. to about 450° C. to drive the chlorine from said CdCl 2  layer into said absorber layers, thereby forming chlorine doped absorber layers, 
 wherein said second p-type absorber layer is formed of CdTe, said third p-type absorber layer is formed of Cd 1-z Hg z Te, where z ranges from about 0.15 to about 0.20, said substrate is continuously conveyed in steps (b)-(g) without being exposed to atmospheric environment. 
 
     
     
       2. The method of  claim 1 , further comprising the steps of depositing a CdCl 2  layer onto said n-type window layer by sputter deposition and then annealing said CdCl 2  layer and said n-type window layer at a temperature in the range of from about 380° C. to about 450° C. to drive the chlorine from said CdCl 2  layer into said window layer prior to the step of depositing said first absorber layer by sputter deposition. 
     
     
       3. The method of  claim 1 , wherein said n-type window layer is formed of CdS or Cd 1-x Zn x S, where x ranges from more than zero to no more than one. 
     
     
       4. The method of  claim 1  wherein said first p-type absorber layer is formed of a p-type semiconductor selected from the group consisting of CdSe, Cd 1-y Mg y Te, Cd 1-y Mn y Te and Cd 1-y Zn y Te, where y ranges from about 0.05 to about 0.20. 
     
     
       5. The method of  claim 1 , wherein the step of depositing said second p-type CdTe absorber layer onto said first absorber layer comprises:
 heating a container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 introducing a heated carrier gas into said container to entrain said CdTe vapor in said heated carrier gas; 
 flowing said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe film on the substrate surface facing said heated conduit. 
 
     
     
       6. A method for fabricating a heterojunction photovoltaic device including three p-type absorber layers in contiguous contact, each of the three absorber layers having a different composition and containing cadmium as a principal constituent, the method comprising the steps of:
 (a) providing an optically transparent substrate coated with a transparent conductive oxide layer thereon; 
 (b) depositing an n-type window layer doped with chlorine onto said transparent conductive oxide layer by sputter deposition; 
 (c) depositing a first p-type absorber layer doped with chlorine onto said window layer by sputter deposition; 
 (d) depositing a second p-type absorber layer doped with chlorine or iodine onto said first absorber layer; and 
 (e) depositing a third p-type absorber layer doped with chlorine onto said second absorber layer by sputter deposition, 
 wherein said second p-type absorber layer is formed of CdTe:Cl or CdTe:I, said third p-type absorber layer is formed of Cd 1-z Hg z Te:Cl, where z ranges from about 0.15 to about 0.20, said substrate is continuously conveyed in steps (b)-(e) without being exposed to atmospheric environment. 
 
     
     
       7. The method of  claim 6 , wherein said n-type window layer is formed of CdS:Cl or Cd 1-x Zn x S:Cl, where x ranges from more than zero to no more than one. 
     
     
       8. The method of  claim 6  wherein said first p-type absorber layer is formed of a p-type semiconductor selected from the group consisting of CdSe:Cl, Cd 1-y Mg y Te:Cl, Cd 1-y Mn y Te:Cl and Cd 1-6 Zn y Te:Cl, where y ranges from about 0.05 to about 0.20. 
     
     
       9. The method of  claim 6 , wherein the step of depositing said n-type window layer doped with chlorine onto said transparent conductive oxide layer by sputter deposition is carried out using an undoped compound target sputtered in a chlorine containing gas environment. 
     
     
       10. The method of  claim 6 , wherein the step of depositing said n-type window layer doped with chlorine onto said transparent conductive oxide layer by sputter deposition is carried out using a chlorine doped compound target. 
     
     
       11. The method of  claim 6 , wherein the step of depositing said first p-type absorber layer doped with chlorine onto said window layer by sputter deposition is carried out using an undoped compound target sputtered in a chlorine containing gas environment. 
     
     
       12. The method of  claim 6 , wherein the step of depositing said first p-type absorber layer doped with chlorine onto said window layer by sputter deposition is carried out using a chlorine doped compound target. 
     
     
       13. The method of  claim 6 , wherein the step of depositing said third p-type absorber layer doped with chlorine onto said second absorber layer by sputter deposition is carried out using an undoped compound target sputtered in a chlorine containing gas environment. 
     
     
       14. The method of  claim 6 , wherein the step of depositing said third p-type absorber layer doped with chlorine onto said second absorber layer by sputter deposition is carried out using a chlorine doped compound target. 
     
     
       15. The method of  claim 6 , wherein the step of depositing said second p-type CdTe absorber layer doped with chlorine onto said first absorber layer comprises:
 heating a container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 introducing a heated carrier gas having a gaseous chlorine constituent into said container to entrain said CdTe vapor in said heated carrier gas; 
 flowing said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor and said chlorine containing carrier gas with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:Cl film on the substrate surface facing said heated conduit. 
 
     
     
       16. The method of  claim 6 , wherein the step of depositing said second p-type CdTe absorber layer doped with chlorine onto said first absorber layer comprises:
 heating a container and a solid CdTe:Cl source material therein to a temperature sufficiently high to thereby create a CdTe vapor and a chlorine gas; 
 introducing a heated carrier gas into said container to entrain said CdTe vapor and said chlorine gas in said heated carrier gas; 
 flowing said chlorine gas and said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor and said chlorine gas with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:Cl film on the substrate surface facing said heated conduit. 
 
     
     
       17. The method of  claim 6 , wherein the step of depositing said second p-type CdTe absorber layer doped with chlorine onto said first absorber layer comprises:
 heating a first container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 heating a second container and a solid CdCl 2  source material therein to a temperature sufficiently high to thereby create a CdCl 2  vapor; 
 introducing a first heated carrier gas into said first container to entrain said CdTe vapor in said first heated carrier gas; 
 introducing a second heated carrier gas into said second container to entrain said CdCl 2  vapor in said second heated carrier gas; 
 flowing said CdTe vapor entrained in said first heated carrier gas and said CdCl 2  vapor entrained in said second heated carrier gas into a heated baffle, thereby forming a vapor mixture of CdTe and CdCl 2  entrained in a carrier gas mixture of said first heated carrier gas and said second heated carrier gas; 
 flowing said vapor mixture entrained in said heated carrier gas mixture from said heated baffle into a heated processing chamber through a heated conduit with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor and said CdCl 2  vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said vapor mixture of CdTe and CdCl 2  with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:Cl film on the substrate surface facing said heated conduit. 
 
     
     
       18. The method of  claim 6 , wherein the step of depositing said second p-type CdTe absorber layer doped with iodine onto said first absorber layer comprises:
 heating a container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 introducing a heated carrier gas having a gaseous iodine constituent into said container to entrain said CdTe vapor in said heated carrier gas; 
 flowing said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor and said iodine containing carrier gas with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:I film on the substrate surface facing said heated conduit. 
 
     
     
       19. The method of  claim 6 , wherein the step of depositing said second p-type CdTe absorber layer doped with iodine onto said first absorber layer comprises:
 heating a first container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 heating a second container and a solid CdI 2  source material therein to a temperature sufficiently high to thereby create a CdI 2  vapor; 
 introducing a first heated carrier gas into said first container to entrain said CdTe vapor in said first heated carrier gas; 
 introducing a second heated carrier gas into said second container to entrain said CdI 2  vapor in said second heated carrier gas; 
 flowing said CdTe vapor entrained in said first heated carrier gas and said CdI 2  vapor entrained in said second heated carrier gas into a heated baffle, thereby forming a vapor mixture of CdTe and CdI 2  entrained in a carrier gas mixture of said first heated carrier gas and said second heated carrier gas; 
 flowing said vapor mixture entrained in said heated carrier gas mixture from said heated baffle into a heated processing chamber through a heated conduit with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor and said CdI 2  vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said vapor mixture of CdTe and CdI 2  with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:I film on the substrate surface facing said heated conduit. 
 
     
     
       20. A method for fabricating a heterojunction photovoltaic device comprising the steps of:
 (a) providing an optically transparent substrate coated with a transparent conductive oxide layer thereon; 
 (b) depositing an n-type window layer onto said transparent conductive oxide layer by sputter deposition; 
 (c) depositing a p-type absorber layer doped with chlorine or iodine onto said window layer; 
 (d) annealing said absorber layer by a plurality of rapid thermal anneal (RTA) lamps at a temperature in the range of from about 450° C. to about 600° C. to thereby increase the grain size of said absorber layer, 
 wherein said n-type window layer is formed of an n-type semiconductor selected from the group consisting of CdS, CdS:Cl, Cd 1-x Zn x S and Cd 1-x Zn x S:Cl, where x ranges from more than zero to no more than one, said p-type absorber layer is formed of CdTe:Cl or CdTe:I, said substrate is continuously conveyed in steps (b)-(d) without being exposed to atmospheric environment. 
 
     
     
       21. The method of  claim 20 , wherein the step of depositing said p-type CdTe absorber layer doped with chlorine onto said n-type window layer comprises:
 heating a container and a solid CdTe:Cl source material therein to a temperature sufficiently high to thereby create a CdTe vapor and a chlorine gas; 
 introducing a heated carrier gas into said container to entrain said CdTe vapor and said chlorine gas in said heated carrier gas; 
 flowing said chlorine gas and said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor and said chlorine gas with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:Cl film on the substrate surface facing said heated conduit. 
 
     
     
       22. The method of  claim 20 , wherein the step of depositing said p-type CdTe absorber layer doped with chlorine onto said n-type window layer comprises:
 heating a first container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 heating a second container and a solid CdCl 2  source material therein to a temperature sufficiently high to thereby create a CdCl 2  vapor; 
 introducing a first heated carrier gas into said first container to entrain said CdTe vapor in said first heated carrier gas; 
 introducing a second heated carrier gas into said second container to entrain said CdCl 2  vapor in said second heated carrier gas; 
 flowing said CdTe vapor entrained in said first heated carrier gas and said CdCl 2  vapor entrained in said second heated carrier gas into a heated baffle, thereby forming a vapor mixture of CdTe and CdCl 2  entrained in a carrier gas mixture of said first heated carrier gas and said second heated carrier gas; 
 flowing said vapor mixture entrained in said heated carrier gas mixture from said heated baffle into a heated processing chamber through a heated conduit with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor and said CdCl 2  vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said vapor mixture of CdTe and CdCl 2  with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:Cl film on the substrate surface facing said heated conduit. 
 
     
     
       23. The method of  claim 20 , wherein the step of depositing said p-type CdTe absorber layer doped with iodine onto said n-type window layer comprises:
 heating a container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 introducing a heated carrier gas having a gaseous iodine constituent into said container to entrain said CdTe vapor in said heated carrier gas; 
 flowing said CdTe vapor entrained in said heated carrier gas through a heated baffle and into a heated conduit connected to a heated processing chamber with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said CdTe vapor and said iodine containing carrier gas with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:I film on the substrate surface facing said heated conduit. 
 
     
     
       24. The method of  claim 20 , wherein the step of depositing said p-type CdTe absorber layer doped with iodine onto said n-type window layer comprises:
 heating a first container and a solid CdTe source material therein to a temperature sufficiently high to thereby create a CdTe vapor; 
 heating a second container and a solid CdI 2  source material therein to a temperature sufficiently high to thereby create a CdI 2  vapor; 
 introducing a first heated carrier gas into said first container to entrain said CdTe vapor in said first heated carrier gas; 
 introducing a second heated carrier gas into said second container to entrain said CdI 2  vapor in said second heated carrier gas; 
 flowing said CdTe vapor entrained in said first heated carrier gas and said CdI 2  vapor entrained in said second heated carrier gas into a heated baffle, thereby forming a vapor mixture of CdTe and CdI 2  entrained in a carrier gas mixture of said first heated carrier gas and said second heated carrier gas; 
 flowing said vapor mixture entrained in said heated carrier gas mixture from said heated baffle into a heated processing chamber through a heated conduit with said baffle, said conduit and said processing chamber being heated to temperatures sufficiently high to prevent the condensation of said CdTe vapor and said CdI 2  vapor onto the surface thereof; 
 heating said substrate to a temperature below the temperatures of said baffle, said conduit and said processing chamber; and 
 conveying said substrate on a path near the exit of said heated conduit to receive said vapor mixture of CdTe and CdI 2  with the distance between the substrate surface and the exit of said heated conduit being controlled to be in the range of from 1 mm to 20 mm, thereby forming a CdTe:I film on the substrate surface facing said heated conduit. 
 
     
     
       25. A modular deposition apparatus for depositing semiconductor compounds containing cadmium, tellurium or sulfur as a principal constituent on substrates to form photovoltaic devices as the substrates are continuously conveyed through the deposition apparatus, comprising:
 a plurality of processing stations connected in series, at least one of said processing stations being a sputter deposition station, at least one of said processing stations being a vapor deposition station; and 
 a conveyor system for conveying the substrates in said processing stations and therebetween, 
 wherein said vapor deposition station comprises: 
 a processing chamber for receiving therein one of the substrates and at least one source material vapor entrained in a carrier gas; 
 at least one vapor deposition source for generating the at least one source material vapor with the at least one vapor deposition source being connected to a baffle, each of the at least one vapor deposition source including a heated container for subliming a solid source material therein to thereby generate one of the at least one source material vapor; 
 a conduit connected to said baffle for directing the at least one source material vapor entrained in said carrier gas onto the substrate surface, thereby forming a film thereon; and 
 a means for heating said processing chamber, the at least one vapor deposition source, said baffle and said conduit to temperatures sufficiently high so as to prevent the condensation of the at least one source material vapor onto the surface thereof.